Techniques for imaging objects have been used for nearly a century in the medical arts for diagnosing and understanding the myriad diseases and maladies that afflict the human body. Imaging techniques have also found use in such diverse fields as radio astronomy, sonar, radar and other fields which require information about an object which is not readily visible to the naked eye and therefore not easily examined. Medical imaging techniques include, for example, X-ray imaging, positron emission tomography (PET), ultrasound imaging and the well known magnetic resonance imaging (MRI).
In all of the imaging techniques mentioned above, narrow band frequency radiation illuminates the object of interest to produce reflected or emitted radiation which is then gathered from the object by a detector. The reflected or emitted radiation is then processed by an imaging algorithm to obtain useful information about the object.
In medical applications, the use of ionizing radiation in imaging, for example with X-rays, involves significant health risks to a patient when the patient is exposed to the radiation for prolonged periods of time or in multiple imaging schemes. Furthermore, certain of these imaging techniques undesirably involve the use of invasive procedures which are both costly and painful. Yet other techniques such as MRI do not yield consistently useful clinical results.
There has thus arisen in the medical imaging art an interest in developing non-invasive, safe and relatively fast techniques which can take advantage of the natural scattering of visible and infrared light through media containing objects to be imaged. Techniques using diffuse light could be used in conjunction with other imaging schemes such as X-ray imaging or MRI to produce highly useful clinical images for diagnostic purposes.
Much of the progress in imaging with diffusive light has focused on ballistic techniques using lasers. With these techniques, an intense pulsed laser illuminates a sample. By time gating photons that have been scattered only a few times and rejecting all other photons, the optical absorption of the medium and objects found therein can be mapped. This technique works best when the allowed time window is short and the photons deviate the least from their "ballistic" trajectory. Unfortunately, the transmittal intensity of unscattered photons diminishes exponentially with increasing sample thickness.
Because of the limitations of ballistic imaging, it is difficult to obtain high quality images of relatively thick objects with low power lasers. Examples of ballistic imaging techniques are disclosed in K. M. Yoo, F. Lie and R. R. Alfano, Optics Letters, Vol. 16, p. 1068 (1991), and in D. A. Benaron and D. K. Stevenson, Science, Vol. 259, p. 1463 (1993).
A second technique for imaging using diffuse light is optical phase modulation. Phase modulation techniques have permitted the location of single absorbers using low power, continuous wavelength lasers. In accordance with these techniques, an amplitude modulated source creates photon density waves that acquire anomalous phase shifts due to the absorber. For the case of a single absorber, the distortions are readily interpreted; however for a more complicated object a general analysis is required.
An example of imaging with diffuse light is disclosed in U.S. Pat. No. 5,119,815, Chance where scattered light was applied to a biological imaging application. The Chance patent discloses a technique for solving the diffusion equation for a homogeneous medium to obtain the overall optical absorption characteristics. This was possible for the homogeneous medium because the long time limit of the logarithmic derivative of the detected intensity yields the absorption characteristics directly. Thus the absorption characteristics for uniform structures may be obtained with the methods and apparatus disclosed in the Chance patent.
Still other attempts to image with diffuse light are disclosed in U.S. Pat. No. 5,070,455, Singer et al. In the Singer et al. system, light intensities are measured at many sensor positions (pixels), initial values of absorption or scattering coefficients are assigned at each pixel, and then a new set of intensities at each pixel is calculated. The calculated intensities are compared to the real intensities, and the intensity differences are used to generate a subsequent interaction of absorption or scattering values for each pixel.
The methods described in Singer et al. usually require many iterations since the absorption or scattering values may not converge rapidly. Furthermore, the Singer et al. system utilizes cumbersome Monte-Carlo statistical techniques which consume large amounts of processing time without guaranteeing computational success. Singer et al.'s methods may also produce false local minima providing misleading results for the absorption characteristics.
Thus prior imaging techniques using diffuse light for scattering fail to solve a long-felt need in the art for robust imaging techniques which can produce reliable images in biological systems. Solution of the aforementioned problems has heretofore eluded the medical imaging art.